1. Field of the Invention
The present invention relates to a pressure control system which can eliminate individual differences of a plurality of pressure controllers used for controlling pressures of a plurality of pressure-controlled sections (or units). The present invention also relates to a substrate holding apparatus for holding a substrate such as a semiconductor wafer to be polished and pressing the substrate against a polishing surface and a polishing apparatus having such a substrate holding apparatus.
2. Description of the Related Art
In recent years, semiconductor devices have become smaller in size and structures of semiconductor elements have become more complicated. In addition, the number of layers in multilayer interconnects used for a logical system has been increased. Accordingly, irregularities on a surface of a semiconductor device become increased, and hence step heights on the surface of the semiconductor device tend to be larger. This is because, in a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, then micro machining processes, such as patterning or forming holes, are performed on the semiconductor device, and these processes are repeated many times to form subsequent thin films on the semiconductor device.
When the number of irregularities on a surface of a semiconductor device is increased, a thickness of a thin film formed on a portion having a step tends to be small. Further, an open circuit is caused by disconnection of interconnects, or a short circuit is caused by insufficient insulation between interconnect layers. As a result, good products cannot be obtained, and the yield tends to be reduced. Furthermore, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after a long-term use. At the time of exposure in a lithography process, if a surface to be irradiated has irregularities, then a lens unit in an exposure system cannot focus on such irregularities. Therefore, if the irregularities of the surface of the semiconductor device are increased, then it becomes difficult to form a fine pattern on the semiconductor device.
Accordingly, in a manufacturing process of a semiconductor device, it becomes increasingly important to planarize a surface of a semiconductor device. The most important one of the planarizing technologies is CMP (Chemical Mechanical Polishing). The chemical mechanical polishing is performed with use of a polishing apparatus. Specifically, a substrate such as a semiconductor wafer is brought into sliding contact with a polishing surface such as a polishing pad while a polishing liquid containing abrasive particles such as silica (SiO2) is supplied onto the polishing surface, so that the substrate is polished.
This type of polishing apparatus comprises a polishing table having a polishing surface constituted by a polishing pad, and a substrate holding apparatus, called a top ring or a carrier head, for holding a semiconductor wafer. A semiconductor wafer is polished by the polishing apparatus as follows: The semiconductor wafer is held by the substrate holding apparatus and then pressed against the polishing table under a predetermined pressure. At this time, the polishing table and the substrate holding apparatus are moved relative to each other for thereby bringing the semiconductor wafer into sliding contact with the polishing surface. Accordingly, the surface of the semiconductor wafer is polished to a flat mirror finish.
In such a polishing apparatus, if a relative pressing force between the semiconductor wafer being polished and the polishing surface of the polishing pad is not uniform over an entire surface of the semiconductor wafer, then the semiconductor wafer may insufficiently be polished or may excessively be polished at some portions depending on the pressing force applied to those portions of the semiconductor wafer. In order to avoid such a drawback, it has been attempted to form a surface, for holding a semiconductor wafer, of a substrate holding apparatus with use of an elastic membrane made of an elastic material such as rubber and apply a fluid pressure such as an air pressure to a backside surface of the elastic membrane so as to uniform a pressing force applied to the semiconductor wafer over an entire surface of the semiconductor wafer.
The polishing pad is so elastic that the pressing force applied to a peripheral portion of the semiconductor wafer tends to become non-uniform. Accordingly, only the peripheral portion of the semiconductor wafer may excessively be polished, which is referred to as “edge rounding”. In order to prevent such edge rounding, there has been used a substrate holding apparatus in which a semiconductor wafer is held at its peripheral portion by a guide ring or a retainer ring, and the annular portion of the polishing surface that corresponds to the peripheral portion of the semiconductor wafer is pressed by the guide ring or retainer ring.
The thickness of a thin film formed on a surface of a semiconductor wafer varies from position to position in a radial direction of the semiconductor wafer depending on a film deposition method or characteristics of a film deposition apparatus. Specifically, the thin film has a film thickness distribution in the radial direction of the semiconductor wafer. Since a conventional substrate holding apparatus, as described above, for uniformly pressing an entire surface of a semiconductor wafer polishes the semiconductor wafer uniformly over the entire surface thereof, it cannot realize a polishing amount distribution that is equal to the aforementioned film thickness distribution on the surface of the semiconductor wafer.
There has been proposed a polishing apparatus for applying locally different pressures to a semiconductor wafer to make the pressing force for pressing a thicker film region on the semiconductor wafer against a polishing surface greater than the pressing force for pressing a thinner film region on the semiconductor wafer against the polishing surface, thereby selectively increasing the polishing rate of the thicker film region. Thus, the overall surface of the substrate can be polished in proper quantities irrespective of the film thickness distribution that has been provided when the film is grown on the semiconductor wafer.
However, when the respective pressures of a fluid such as pressurized air supplied to respective pressure chambers positioned on the reverse side of the semiconductor wafer are independently controlled so that the pressure applied to the semiconductor wafer for every zone (region) is controlled, it is necessary that a plurality of pressure controllers which are the same in number as the pressure chambers are installed with a one-to-one correspondence and the pressures of the respective pressure chambers are controlled at desired values by the respective pressure controllers. In this case, each of the pressure controllers can perform feedback control for itself, but cannot perform any control between itself and other pressure controllers. Specifically, each of the pressure controllers cannot eliminate an individual difference between itself and other pressure controllers. Therefore, even if the fluid having the same pressure is expected to be supplied to the respective pressure chambers by controlling the respective pressure controllers, the respective pressure chambers cannot be kept at the same pressure because pressures outputted from the respective pressure controllers are different from each other by the individual differences of the pressure controllers. Accordingly, the semiconductor wafer cannot be polished uniformly over the entire surface thereof.
Further, even if a predetermined differential pressure is expected to be developed between the two pressure chambers to make a pressing force for pressing a thicker film region on a semiconductor wafer against a polishing surface greater than a pressing force for pressing a thinner film region on the semiconductor wafer against the polishing surface, thereby selectively increasing the polishing rate of the thicker film region, the predetermined differential pressure cannot be developed between the two pressure chambers because pressures outputted from the two pressure controllers are added by pressure errors caused by the individual differences of the pressure controllers. As a result, the respective zones (regions) of the semiconductor wafer cannot be polished at desired polishing rates.
In the above example, the individual differences of the pressure controllers are described in the case where the pressure controllers are incorporated in the polishing apparatus. However, in the case where pressures of a plurality of pressure-controlled sections (or units) are controlled using a plurality of pressure controllers, the same problem arises due to the individual differences of the pressure controllers. Specifically, pressures of the respective pressure-controlled sections cannot be controlled to desired values owing to the individual differences of the respective pressure controllers.